Wavelength division multiplexing (WDM) technology may multiplex more than one carrier signals in a single optical fiber. Each carrier signal has different wavelength. Carrier signals may be combined by a multiplexer (Mux). The combined carrier signals may travel in a fiber. The combined carrier signals form a multiplexed signal. After leaving the fiber, the combined carrier signals may be separated into the original carrier signals by a demultiplexer (Demux).
Mux and Demux may be constructed using various approaches, for example, Bragg grating approach, fused fiber coupler approach, and the like. An approach is based on a set of thin film filters (TFFs). A first TFF may transmit a first wavelength, e.g., λ1, and reflect all other wavelengths, e.g., λ2, λ3, and λ4. Thus, a first carrier signal having wavelength λ1 may be selected. A second TFF may transmit a second wavelength, e.g., λ2, and reflect the remaining wavelengths, e.g., λ3, and λ4. Thus, a second carrier signal having wavelength λ2 may be selected. A third TFF may transmit a third wavelength, e.g., λ3, and reflect the remaining wavelength, e.g., λ4. Thus, a third carrier signal having wavelength λ3 may be selected. Finally, the last carrier signal having wavelength λ4 remains and thus it may be selected as well.
A free-space WDM Demux using TFFs is disclosed in U.S. Pat. No. 5,583,683 to Scobey. Scobey teaches that the multiplexed carrier signals containing wavelength λ1-λ8 exit from a common fiber coupled with a fiber collimator. The multiplexed carrier signals are collimated by the fiber collimator and propagate in free space until impinge on a first TFF. The first TFF transmits the first carrier signal having wavelength λ1 and reflects the remaining carrier signals having wavelengths λ2-λ8. The transmitted collimated carrier signal having wavelength λ1 is coupled to a fiber channel 1 by a fiber collimator.
The reflected remaining carrier signals impinge on a second TFF. The second TFF transmits the second carrier signal having wavelength λ2 and reflects the remaining carrier signals having wavelengths λ3-λ8. The transmitted collimated carrier signal having wavelength λ2 is coupled to a fiber channel 2 by a fiber collimator. The process is repeated until the last TFF transmits the carrier signal having wavelength λ8. The device can also function as a Mux when the light path is reversed.
A compact free-space WDM Mux/Demux using TFFs is disclosed in U.S. Pat. No. 8,538,210 to Wang et al. Wang et al. teaches that the fibers and their corresponding fiber collimators are disposed on two opposite sides' of a substrate. Both Scobey and Wang et al. use fiber collimators. A fiber collimator typically has a diameter larger than 1 mm. Thus the sizes of Muxs/Demuxs of Scobey and Wang et al. may not be reduced, since the sizes are limited by the size of fiber collimators used. Accordingly, new approaches to further reduce the size of free-space WDM Mux/Demux are required.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention.